1. Field of the Invention
The invention concerns a linear micrometer designed by the Abbe principle with a measuring spindle sleeve metering into a housing and a scale arranged within the housing in a stationary manner and a readout device correlated with the scale and coupled with the measuring spindle sleeve to indicate measured distances, preferably by digital electrical means.
2. Description of the Prior Art
A linear micrometer is a device for the measurement of small lengths and distances wherein the measuring spindle sleeve is driven by means of crossed helical gears with a slight pitch and the measuring means is a linear scale. Micrometers having an extended measuring range and equipt with rapid setting means which may be released from the helical gears are known (for example, as described in German Offenlegungsschrift No. 16 23 170).
Observation of Abbe's principle requires that the measuring means constitute an extension of gage length, i.e. the axis of the measuring spindle sleeve and the measuring means, are aligned with each other. Concerning the measurement of the displacement of the measuring spindle sleeve, it is known especially in the case of digital electrical distance acquisition to either couple the measuring means with the measuring spindle sleeve and to move it past the stationary readout device, or to couple the readout device with the measuring spindle sleeve and to move it past the stationary scale (as shown in U.S. Pat. No. 2,592,264). The last-mentioned arrangement has the particular advantage of a shorter length of the device, but also poses higher requirements for the accuracy of the guidance of the measuring spindle sleeve (see for example, K. Rantsch "Optics of Fine Measuring Techniques" (1949, page 191.)
A longitudinal distance measuring device is taught in German Auslegeschrift No. 26 05 020 having a scale solidly mounted at its two ends within a housing. The readout device is displaced on a straight line slide bar arranged parallel to the scale by means of a cable line. To maintain a constant reading angle, an additional rotation stop is provided for the readout device. The measuring spindle sleeve is resolved into three rods secured to the readout device and emerging from the housing through three bores. Outside the housing the three rods are maintained together by means of a plate wherein a probe tip is set, the probe being aligned with the axis of the scale. The measuring accuracy in this case depends on the accuracy of the guidance of the readout device relative to the scale. Because the rods forming the measuring spindle are without guidance over their full length, they constitute a relatively long lever around which the probe tip may be rotated out of the axis of the scale, thus violating Abbe's principle.